In order to mimic the human metabolic pathway of cyclosporin A (CyA) a total of 28 bacterial and 72 fungal strains was screened for their ability to transform CyA. Among 3 bacteria and 11 fungi, which produced the main human metabolite OL-17 [ηHyMeBmt¹]CyA, Actinoplanes sp. (ATCC 53771) achieved the best transformation rate (5.4%). Furthermore, the two N-demeth-. ylated minor products [Leu⁴]CyA (3.2%) and [Leu⁹]CyA (4.7%) were isolated, both known as minor natural metabolites and the first one also as a human biotransformation product. Microbial conversion of CyA using the actinomycete Sebekia benihana (NRRL 11111) yielded [yHyMeLeu⁴]CyA (35%), [γHyLeu⁴]CyA (4.5%) and [γHyMeLeu⁴, γHyMeLeu⁶]CyA (8.6%). The structures of these derivatives correspond with those of the human metabolic pathway. The related compounds [Nva²]CyA (CyG) and [D-MeSer³]CyA were similarly converted to the corresponding 4-γ-hydroxylated analogues. None of the biotransformation products showed a better immunosuppressive effect than CyA, although in various cases the cyclophilin binding affinity was comparable to that of CyA.